The auxiliary use of oxy-fuel combustion burners is well known in glass melting furnaces. On furnaces operating conventionally with air, an oxy-fuel combustion burner or a limited number thereof are then added. The purpose of introducing these additional burners is generally to increase the capacity of existing furnaces, possibly when their performance is seen to decline owing to their old age. This situation is found for example when the regenerators associated with such furnaces have become degraded and are no longer able to heat the air used for combustion sufficiently. The capacity of a given furnace may also simply be increased by introducing additional energy sources.
As a general rule, the additional oxy-fuel combustion burners are placed close to the zone in which the batch materials are charged into the furnace. These burners thus melt the batch materials. The addition of a few oxy-fuel burners in large-capacity furnaces is usually accomplished without any substantial modification to the general operation of the furnace in the sense that, in particular, the regenerators continue to operate and therefore handle both the combustion flue gas arising from burners operating with air and that arising from burners operating with oxygen.
Beside the fact of having an additional energy source, these systems operating in what is called “oxy-boosting” mode do not provide the benefit of all the known advantages that may result from oxy-fuel combustion. Among the number of potential advantages are mainly a lower energy consumption and reduced emissions of undesirable flue gas.
Oxy-fuel combustion provides an energy saving at least for the reason that the energy of combustion gas is not partly absorbed by the nitrogen of the air. In conventional furnaces, even if some of the energy carried off with the nitrogen is recovered in the regenerators, the flue gas finally discharged still gives off a significant amount of energy. The presence of nitrogen contributes to this loss.
Reducing the energy consumption by a production unit in question has in addition the advantage of consequently limiting the carbon dioxide emissions and therefore of meeting the statutory requirements in this field.
The presence of nitrogen is also a source for the formation of nitrogen oxides, called NOx, the emission of which is practically prohibited because of damage due to the presence of these compounds in the atmosphere. In practice, users strive to operate furnaces under conditions leading to emissions that are as low as possible. In the case of glass furnaces, these practices are not sufficient to meet the very stringent standards in force, and it is necessary to carry out an expensive flue gas decontamination operation by the use of catalysts.
By using oxygen, it is possible to circumvent the problems associated with the nitrogen in the air, something which is not the case in oxy-boosting techniques.
Despite the abovementioned advantages, the use of oxy-fuel combustion in large glass furnaces remains to be developed. The reasons for this are of several types. Firstly, the use of oxygen is necessarily more expensive than that of air.
The economic assessment of the use of oxy-fuel combustion is positive only if it is possible to recover a significant amount of heat from the flue gas. Hitherto, recovery of this energy does not seem to have been accomplished satisfactorily and the potential energy saving has not been actually achieved.
Moreover, the implementation of oxy-fuel combustion still poses technical problems that counteract certain advantages. One recognized difficulty is due to the corrosion of the refractories, this corrosion reducing the lifetime of the silica refractories of the furnace roof. This is because the high H2O content of the combustion atmosphere causes two deteriorating effects:                the first is due to the diffusion of H2O into the glassy phase of the refractory blocks; and        the second is due to the condensation of sodium hydroxide present in the atmosphere on the refractory bricks, which entails a high degree of oxidation, in fact six times higher in the case of an oxy-fuel combustion furnace.        
To obviate these conditions, it is necessary to use materials that are more corrosion-resistant than those normally chosen. Usually, for various reasons, the roof of large glass furnaces is made of silica bricks. In the case of an oxy-fuel combustion furnace, it is necessary instead to use materials such as alumina, AZS or spinels. However, these materials are more expensive and also pose problems because they are significantly heavier.
Further, new problems have also appeared in practice that require specific new operating conditions to ensure that this technique be effectively used in applications that the theory suggests would be advantageous. The invention relates to ways of implementing the oxy-fuel combustion technique in large glass furnaces that form the subject matter of the claims appended to the present description.